It just idles a little rough but runs like a boss. It fluctuates a couple hundred rpm up and down in little bumps with no input from me. Only slightly hesitant to start, usually on the second bump with just a bit of choke. Doesn't stall once I'm out, I keep it at about 12-1500rpm cause I'm new at this and... rough idle. Oh, it's a Mikuni pumper carb, 351cc Bill Blue.

Previous owner said he rode often, and evidence shows that is true (lots of leaves, sticks and mud in odd places), but it obviously didn't get a lot of attention. Filter and airbox were filthy.

Napa site says Seafoam spray might be good for a rough idle, like this: " Sea Foam Spray is used by inserting the included application hose and patented “HOOK TOOL” into the air intake just in front of the throttle plates in the throttle body. Then start engine and let it warm up to operating temperature. Increase idle speed to 2,000 RPM and spray the contents of the container into the engine. Shut the engine off and allow it to “Hot Soak” for 15 minutes. After the Hot Soak period restart the engine and road test the vehicle aggressively until the exhaust is clean (road test normally takes 5 to 10 miles of driving). "

sound reasonable? Good place to start?

posting a picture of something completely unrelated, just for fun. Is that ok?


OFW Skate Park, Atl 2017

 

Sounds like you need to adj the air fuel low speed 't' screw on the bottom of the carb. Turn it in/out and center in the highest rpm range after warmed up.
Are you in the same elev as the original owner?
I definitely would do that before using any 'product'.

 

I'm about 900 feet lower. The guy that did the 351 bore kit and pumper carb is at the same altitude as me. I'll look to that adjustment after I get the wheels/tires back on. Hopefully by tomorrow night.

Thanks... t

 

Nice pic!

I'm not afraid of seafoam. I'd run a good dose in the tank and try the hot soak described. But also get the filter clean and work on the adjustment durielk describes.

Seafoam can help clean a little crud out of the carb, but it is not a miracle product and sometimes things need to come apart for a cleaning.

 

Meh...I'm not a big fan of a spray fuel cleaner as much as a full liquid additive. Instead of Sea Foam I'd recommend Chemtool B12 in the 12oz. or so can. Pour just a little more than recommended into the fuel tank and drive the bike normally through that tank. Don't do a bunch of high rpm or performance runs because you've probably reduced the octane rating of the fuel in your tank by a little. Just ride the bike in a "more normal" manner.

This stuff doesn't hurt rubber carb parts or other metallic parts, but it is a fairly harsh cleaner so just keep it off your hands. Another method I've used in addition to adding B12 to the tank is to introduce it straight into the carb. Drain the carb at the bottom of the bowl with the fuel off. Now fill the carb with a squirt bottle or other device through the fuel line until it overflows from the open end of the fuel line. Wear some nitrile gloves and eye protection and go slowly. Let this cleaner sit in the carb overnight at least. When restarting the bike you don't have to drain the B12 though starting will probably be a little slower the first time. This small amount will quickly dissipate through the engine without harm. I've had this effort even clean a plugged pilot jet occasionally which is nearly impossible to do without removing the carb and pilot jet.

All of this is assuming that some dirt or residue in the carb is the main culprit. As others have suggested you may just have an idle mixture adjustment screw issue or maybe even the need to do some internal rejetting. Do you have Bill's 34mm pumper or the 36mm aftermarket pumper?

 

It's a Mikuni from Bill. I'm pretty sure. :^

 

 

I have to agree with TNC(damn, that hurt...) about the spray-it won't hurt but it aint gonna help much either. Seafoam and B-12 are essentially the same type of cleaner, in that they need to flow through the fuel circuits to clean them so its best to add to the fuel. Sounds like you may need to adjust the idle mixture. and give me a head up if you venture up to my neck of the woods.

 

your neck of the woods is on the short list.

I think Sea foam is in the cards. After I get the airbox sorted. Just got the super knobbies off, some decent 50/50s on, and now need to address the uneven idle and get some miles in.
Or get miles in and fix the idle... just all of the above/and more of what I just said... t

 

Sounds like a plan. Just remember that it gets SLIPPERY as hell, so plan accordingly.